The Launch Pad HARM AGM-88 CG / CP Question

I picked up a TLP HARM kit a while back and am just getting around to building it. I would like to find where CG/CP needs to be for this kit, or to otherwise be enlightened on how to determine this. I'm not experienced with either RocSim or OpenRocket - I've found a couple of HARM RocSim files, but they indicate that they were scratch-built, so I have no idea how the materials chosen for those might compare to the TLP kit.

Starting from scratch in OpenRocket seems difficult, because I have no idea as to the parts that TLP uses for this kit, especially when it comes to the nose cone.

I'm probably over-complicating this, but I want this one to fly true; I plan on putting a lot of time into the finish and don't want a crack-up if I ever should fly it (it would seem a shame not to).

The commercially available decals seem really good, but I may try making my own to more accurately reflect the real thing - maybe a mix of the two. Also going to shoot for some more realistic leading edge fin treatments, and at some point, it would be neat to replace the TLP nose cone to more accurately reflect the 88's ogive, too, but I doubt I'd be able to find one.

I spent a lot of years on the actual HARM program, so I've got a soft spot in my heart for this bird (traveled the globe supporting the thing - at one point touched/tested every one in the Air Force inventory, so I pretty much know every bolt, hatch, and designation used on the airframe, including CG - would that point translate to a scale model?).

Either way, this is a gorgeous kit. I'm probably not being realistic as to my building skills versus the vision I've got in my head, but I'd be ecstatic if mine turned out nearly as beautifully as the ones sodmeister, rifleshooter, and others who have posted their builds on here have done.

As far as flying, the Aerotech E15-4 should be adequate as long as I don't deviate from plan too much (e.g. nylon chute & nomex wadding), correct?

Thanks for any input you all might be able to share!

Interesting you should ask. I bought their Maverick AGM-65B kit and nowhere in the instructions does it mention where the CP or CG should be. I always thought it was strange that someone would produce a kit and leave out the 2 most critical pieces of information.

I know this doesn't help you, but I thought it was maybe just the kit I bought.

I've seen posts and videos about the TLP kits not flying well, but it's probably due to a lack of this info. I emailed TLP directly to inquire about both our kits. I'll let you know if I hear anything back.

-DBB

I'm very interested in their response as to why they left out the information.

Afaik CP is proportional. So it scales. So if you know the CP for one size, you can figure it out for all others.

OTOH if you give me the length of pieces and the rough sizes of fins it can be worked up pretty quickly. CG is irrelevant if kept ahead of CP by enough... so knowing the CP will do.

As for why he does not give them, perhaps when he designed them he simply experimented until they flew well. Many TLAR designs are done this way. Iteratively. Perhaps he simply knows if you follow the instructions to a tee, it will fly. All conjecture on my part.

Last time I check MOST manufacturers don't give CP or CG
Cheers

"When Gortatowsky talks TLP CP, people listen." Now that is an line from a commercial in the 70's, man I am getting old.

Right there with you, Daddyisabar. E.F. Hutton, wasn't it? LoL

Thanks much, Gdjsky01! Haven't heard back from TLP yet... I'm building stock, so it will be interesting to see where CG comes in with relation to your RockSim calcs. You didn't happen to save that file, did you..? If so, would you mind posting it?

i really need to start playing with OR or RockSim to get a handle on this stuff.

-DBB

I'll be honest, all I did was use the scale function in ROCKSIM. All that does is scale the width and length dimensions. So all the material values for thickness and mass remained the same. So I had a 65% scaled version that still weighed what the 4 inch diameter rocket weighed.

In other words, I did not take the time to replace the actual materials used in the file. So the CG in the file is useless. Only the CP is useful which is why I posted it. Remember I gave you the CP not the CG.

If you hear back from TLP I'd be shocked... but in a good way.

Jeff

Well the three TLP kits I have flow all flew very well; though I did make significant changes in the materials used on two of them, I didn’t change the actual shape/configuration of either.
The changes probably moved the center of gravity towards the rear of the rocket so I did add some nose weight to offset that.
The way I look at it; if Giant Leap’s “Talon” or “Crossbow” series of rockets fly well then I figure all those odd TLP designs will as well. You know; big fins placed well forward, itty-bitty fins at the rear.

Most of the issues crop up because they were designed to be light weight. Many were only designed with the D12 in mind. Many are strange to model without active flight stabilization the real rounds have. Then people modify them, or over build them, they fly like crap, and they blame TLP. This has been an ongoing issue for YEARS. And I mean YEARS. Chuck B of TLP used to be active on rec.models.rockets and other early forums. But IIRC he gave up on online forums because of the constant badgering about 'quality' or 'flew terrible'. Almost everyone did not follow his directions or built heavy (Chuck also had health issues.) I am not even sure if he is still with us or with TLP.

Boomtube I am glad your's fly well! You knew what you were getting into and changed the weight accordingly. :clap:
I have had the Osiris MPM, the Matra 530, and the Hawk. I have about 10 :y: more in the build pile. No... I just counted... try 16!! :y: :y:

Not to hijack the thread, tho i think its been answered, here's the Osiris MPM flying earlier this year on a F39. It's box stock and built in 1998... yep '98

Thanks, Jeff. And everyone else who replied, too.

Have not heard back from TLP...


Your point is well taken regarding overbuilding, then complaining that they don't fly we'll. To be honest, I try to take negative comments in forums with a grain of salt, laced with some of my own judgement (which is probably faulty, as well). Honestly, if you dig hard enough on any forum, you'll most likely find multiple conflicting "facts" on the same topic, which makes it hard for the casual hobbyist like me.


The one build thread I found for the TLP HARM used nomex wadding and nylon 'chutes, which I found interesting and something to consider, but that would definitely change weight characteristics, I would think.


Regardless, The TLP kits look great to me. i haven't seen anyone else offering the breadth of missile kits that they have. I may just end up keeping it as a model and not fly it at all.


Think I'll play around with OpenRocket again and see if I can achieve the scaling result, or maybe try and start a sim from scratch.


Wish me luck, and thanks again..!


DBB

Find a rocket in a simm with BT and fins that are similar. Start there and then tweak the simm to be what you have and go from there.

DBB said:

Thanks, Jeff. And everyone else who replied, too.

Have not heard back from TLP...


Your point is well taken regarding overbuilding, then complaining that they don't fly we'll. To be honest, I try to take negative comments in forums with a grain of salt, laced with some of my own judgement (which is probably faulty, as well). Honestly, if you dig hard enough on any forum, you'll most likely find multiple conflicting "facts" on the same topic, which makes it hard for the casual hobbyist like me.

Click to expand...

Did I “Overbuild” my two larger TLP kits? You betcha, as I have absolutely no faith in Estes OEM BT-80 tubes. Especially long Estes BT-80 tubes and most especially long BT-80 tubes that are composed of two shorter tubes with a coupler. To put it bluntly I think they have all the rigidity and structural integrity of an empty aluminum beer can. I have purchased four different kits, based on Estes OEM BT-80 tubes, from three different manufactures and they all arrived with one or more of the big BT-80 tubes damaged in some manner or another. The vendors were good enough to send me replacement tubes which, you can see it coming, all arrived smushed.
Now the packages the kits and replacement tubes were shipped in were not damaged in any manner, which begs the questions; when, where and how were the tubes damaged in the first place if not during shipment? I don’t know the answers to those questions and really don’t care, I just don’t plan to order anymore of these types of kits unless I plane to replace the OEM tubes with something stouter.
I also trust big balsa fins about as much as I trust what is coming out of the media these days. The quality of balsa nowadays varies from, hard and dense as plywood, to sheets that resemble caked flour. You can’t even sand the stuff without it turning to dust in your hands; you can’t even hold the stuff without leaving dents in it. I once had some balsa fin stock from one kit that actually melted when I applied the first coat of wood sealer.
So yes I overbuilt my “Scimitar” and “Perseus II” using BMS heavy wall tubes and Bass Wood fins. Knowing they would be heavier than stock models and wanting a wider range of motor options I went with 29mm motor tubes along with AeroPack retainers.
Naturally I put some additional weight in the nose and both models have flown successfully and very well.
Don’t get me wrong, I love the look of the TLP kits but I personally don’t understand the thinking behind building large-sized “Mid-Power” rockets out of the same materials used to build Estes Alpha C6-5 rockets. But that’s just my opinion and others obviously disagree.

I ordered directly from Estes, and receved crushed tubes, that they crushed when they packed the box. :eyepop: The box was not damaged. Needless to say I don't trust thier packing, plus they are being shipped half-way around the world from China, before it leaves Colorado! :y:
I am currently building a TLP Perseus 1.6", and is mostly stock. I used the coupler as a ejection baffle, with a eye bolt for a parachute mount. Papered the fins, my only complaint was the rear fin stock was warped, I was able to flatten the sheet to make it usable. I have the TLP AGM-88 on que as a future build, and will watch this post to see what happens. op:
Good Luck

boomtube said:

Did I “Overbuild” my two larger TLP kits? You betcha, as I have absolutely no faith in Estes OEM BT-80 tubes.

Click to expand...

The decision to deviate from build plans, whether to reinforce a kit for some of the shortcomings you mentioned, or to simply add custom touches is at the root of most hobbies, and certainly adds to the enjoyment of it. But it is with the flying hobbies that experience, skill, and good judgment come into play to achieve a successful flight. In this case, knowing how to compensate with weight and/or proper engine selection, etc... Not having this experience and being tempted by the thrill of the build is probably what gets us casual hobbyists into trouble, and could very likely account for a large number of the negative posts I mentioned, for any kit.

i can certainly understand the need to vent after spending hours and hours building and finishing a kit, only to see it go horizontal off the rod to its demise.

I received an Estes Saturn kit with a crushed BT, in an undamaged box, so it seems I'm not alone. The HARM is also two BT-80 tubes joined with a coupler, but they seem fairly sturdy. I've never flown anything greater than an Estes D, and so don't have the experience to know how heavy it would need to be to withstand higher power flight.

if I decide to fly this, it would probably be with an E15-4 (which from what I understand is at the low end of the mid-power scale?), simply because I've read so much about these larger models - the Saturn included - not doing well on the D engines recommended by the manufacturer.

(Final build weight and engine selection will probably be another post...).